The invention relates to an interferometric measuring device capable of profiling a surface with large height variations.
Conventional phase shifting interferometers require that the surface of an object being profiled be quite smooth, so that continuous interference fringes are produced by it. A large step change (i.e., a quarter of a wavelength of the light used to make the measurement or more) in the height of the surface often destroys the continuity of interference fringes, and consequently conventional phase shifting algorithms executed by a computer in response to fringe intensity data produced by a solid-state imaging array, such as a CCD array, are unable to accurately compute the profile of the surface.
At the present time, measurement of accurate profiles of surface areas is limited to RMS average roughness of approximately one thousand Angstroms using single wavelength interferometric techniques. Using multiple wavelength techniques (such as those described in commonly assigned U.S. Pat. No. 4,832,489, issued May 23, 1989, to Wyant et al.), surfaces with approximately one micron average roughness may be measured. With single wavelength techniques, the present state of the art limits measurement to surface step features of no greater height than approximately 0.16 microns. With multiple wavelength techniques, step height measurements are limited to steps less than approximately 15 microns in height.
U.S. Pat. No. 4,818,110 (Davidson) discloses a Linnik Microscope in combination with a video camera, a wafer transport stage, and data processing electronics, based on the use of an interference microscope to measure height and width of surface features on an integrated circuit. However, this reference does not disclose pixel-by-pixel mapping of the surface of a sample, does not generate a profile, and is incapable of generating an accurate pixel-by-pixel area profile of a surface that is too "rough" to be measured by conventional interferometry.
The article "Profilometry with a Coherent Scanning Microscope", by Byron S. Lee and Timothy C. Strand, Applied Optics, Volume 29, No. 26, Sep. 10, 1990, discloses a "coherence scanning microscope" in which an object is scanned in the z direction. White light interference fringes that result from the scanning are demodulated to find the peak amplitude of an envelope of the fringes to determine the value of z at the peak interference fringe. The Lee and Strand paper discloses no specific way of demodulating the fringes, and indicates that ambiguities introduced by phase change on reflection due to dissimilar materials renders the technique inoperable. No interpolation techniques or curve fitting techniques that might improve accuracy are disclosed. Optical path difference increments apparently are limited by the step size of stepper motors used, as is the speed of incrementing. The disclosed profile data is two-dimensional, rather than three-dimensional. The Lee and Strand reference clearly does not teach a technique to accomplish fast, highly accuracy surface profiling of surfaces having wide ranges of smoothness or roughness, or of dealing with phase ambiguity errors that result from phase change on reflection due, for example, to dissimilar surface materials.
Although phase-shifting techniques can produce measurements of surface roughness of the order of one thousandth of a wavelength, most present methods detect phase modulo 2.pi., and consequently give rise to errors sometimes referred to as "2.pi. ambiguities" but hereinafter referred to as "phase ambiguities" or "phase ambiguity errors". Various kinds of "phase unwrapping" algorithms are used to track the phase over a large range of surface heights and resolve the phase ambiguity errors. Problems arise when there is a height variation between two adjacent pixels that cannot be unambiguously "unwrapped". The result is an integration error that usually manifests itself as a streak across the field of view.
It is well known that different materials on a surface to be profiled produce a phase shift known as "phase shift on reflection", which introduces phase ambiguity errors when conventional phase shifting techniques are utilized to determine the surface profile. More specifically, it is known that if the material of a surface being interferometrically profiled has optical properties such that the incident ray is delayed in phase by an appreciable amount, there will be a shift, i.e., by the "phase shift on reflection" in the phase of the fringe pattern received at the detector. Phase shifts which can cause phase ambiguity errors also may occur when there is a thin transparent film on the surface being optically profiled, because the film adds delay to the light propagation time therethrough.
There is an unmet need for an accurate, high speed, noncontact profiler capable of profiling a wide variety of rough surfaces.